In HPLC, a sample to be examined must be fed into a high-pressure liquid stream which is then supplied to the chromatography column. The chromatography column is normally kept at a constant temperature, which may amount to up to 110° C. and in the near future even up to in the region of 150° C., in a column oven. As a result of the temperature control of the column, a higher value for the flow through the column is attained at a given pressure. This yields shorter processing times.
To attain short capillary paths between a sample input device, the switching valve for the introduction of the sample volume into the fluid stream and the column, it is desirable for the switching valve to be positioned as close as possible to the column, preferably on or in the column oven. The switching valve must accordingly exhibit relatively high temperature resistance.
Whereas the purely mechanical components of a switching valve, in particular of a high-pressure switching valve for HPLC, have no problems or few problems with regard to temperature resistance, problems are encountered in attaining the desired temperature resistance of electronics components, in particular for the detection of the rotational position of a mechanical part of the switching valve.
Switching valves for introducing a sample into the fluid stream normally have a stator in which there are provided multiple connection ports for the supply and discharge of the fluid to and from the switching valve. The ports are connected via ducts to opening cross sections which are formed on a switching surface of the stator, for example in the face side of a substantially cylindrical stator element. The rotor likewise has a switching surface which interacts with the switching surface of the stator, wherein in the switching surface of the rotor there are formed grooves which serve to connect certain opening cross sections and/or ports of the stator to one another as a function of two or more switching positions. Here, the rotor and the stator must be pressed against one another with an adequately high pressing force in order to attain a sealing action in the plane of the switching surfaces even in the case of high pressures such as arise in liquid chromatography, in particular HPLC.
Such switching valves are described for example in WO 2009/101695 or US 2010/0281959 A1.
In the case of valves constructed in this way, it is necessary for the position of the rotor, which is normally driven in rotation by a suitable drive unit, for example an electric motor with a gearing, to be detected with sufficient accuracy in order that the switching positions formed by the rotor and the stator can be moved to with sufficient accuracy.
For this purpose, known switching valves have electronics for detecting the position of the rotor. Said electronics are normally arranged spatially in the vicinity of the rotor. Here, the position of the rotor may in particular be detected optically. Here, optical marks are provided on the rotor itself, which optical marks can be detected by means of a photodetector, for example a photodiode.
The use of such photoelectric or electric (for example capacitive) sensors however poses problems at temperatures considerably higher than 70° C. For this reason, such switching valves cannot be positioned entirely in the column oven, at least not with the switching valve head in which the rotor and the stator are provided. This is however desirable because switching valves are commonly arranged in a wall of the column oven, wherein the column oven for this purpose has corresponding apertures in the wall, into which apertures in each case one switching valve can be inserted. Here, the switching valve head is situated on the inner wall of the column oven or projects into the interior of the column oven so as to permit the connection of capillaries to the respective ports of the switching valve.
To solve said problem, it is known for the movement of the rotor to be limited by means of stops, such that the switching position which is delimited by the stops can be moved to in an exact manner. This is however possible only in the case of switching valves which have two switching positions. Furthermore, it is possible for the mechanical stops to be used for the calibration of the position detection device, in order hereby to allow for temperature-induced deviations. In the case of such valves, it is thus not necessary for position detection by means of a corresponding sensor device to be provided directly at the rotor.
However, in the case of such switching valves, it is necessary to carry out calibration processes which entail a corresponding amount of effort, in particular are time-consuming.